This invention relates to an interferometer for measuring the wave front qualities of components used in optical data storage systems and more particularly, but not by way of limitation, to an interferometer which can be used to measure wave front qualities of collimated laser sources such as collimator pens having semiconductor diode lasers. Also, the invention can be used as a guide for alignment of optical components such as the collimating objective lens and cylindrical lens in the manufacturing of collimators pens.
In optical data storage systems, a light beam from a coherent light source such as a gas laser or semiconductor laser is focused on a spot having a diameter of 1 micrometer or less. This intense light beam is used to ablate holes on metallic thin films such as telurium coated on a substrate material as a means for storing digital information for computer applications. In order to obtain a small focus light spot to record information, optical components used to shape the light beam must have high optical qualities. Therefore, it is necessary to use an instrument to qualify these optical components before they are put into an optical data storage system.
In the testing of optical components for optical disc drives there are two requirements. One is to find a practical standard for acceptable optical quality. The second is to find test equipment that can perform proper measurements. The quality of an optical surface is usually defined by its closeness to an ideal surface. Therefore, in optical testing, an interferometer is used to compare the optical surface under test with a test plate. The difference between the two surfaces shows up as a distortion in interference fringes. By means of a computer, a two-dimensional plot can be extracted of the phase difference between the two surfaces from their interference fringes. Also, a figure of merit cna be obtained in the terms of its RMS (i.e. root mean square), deviation from the test plate. Although a phase plot of the surface gives the information about its surface, it is not easy to examine the phase plot and predict its performance in a system.
Currently, two different types of interferometers are used to check the optical qualities of incoming optical components. One interferometer is used to test mirrors, beam splitters and other objects under test. The second interferometer is built to test laser pens. A toric lens used in an optical subsystem is tested in a different set-up for the quality of its astigmatic focal lines. It is time consuming to use these two interferometers to measure the qualities of optical components. Moreover, the results of the tests are not in a form that an inexperienced operator can use readily to qualify optical components.
It has been found that the optical subsystem in the optical disk drive can be a critical tester for optical components. The optical subsystem has similar properties as an interferometer. The pregrooves on the optical disk split the incoming beam into three beams so that the beams upon returning through the objective lens interfere and generate radial push/pull signals at the location of a quadrant detector. Any asymmetry in the wave front incident on the disk can cause the signals at each detector on the quadrant detector to be out of phase with respect to one another. This produces crosstalk in the focus error signals. Phase differences as high as 80 degrees have been observed. It has also been observed that when the phase difference was about 20 degrees the crosstalk was less than 5 percent. The fact that a 20 degree phase delay can be measured in signals from the quadrant detector means that the phase asymmetry can be measured in the wave front as small as 1/18 of wavelength of light. Therefore, an interferometer based on the same concept as the optical subsystem in an optical disk drive can be a powerful test instrument for qualifying incoming optical components. This conclusion led to the subject invention as described herein.
In the following United States Patents, U.S. Pat. No. 3,829,219 to Wyant, U.S. Pat. No. 4,474,467 to Hardy, et al, U.S. Pat. No. 3,904,295 to Hock, et al, U.S. Pat. No. 4,180,830 to Roach, U.S. Pat. No. 4,236,823 to Roach et al, U.S. Pat. No. 4,340,305 to Smith, et al, U.S. Pat. No. 4,363,118 to Roach, et al, U.S. Pat. No. 4,492,468 to Huignard et al and U.S. Pat. No. 4,344,707 to Massie, various types of shearing interferometers and wave front sensors are disclosed. None of them provide the unique features and advantages of the subject wave front quality measurement device as described herein.